Real-time isolation of self-test and linear acceleration signals

ABSTRACT

A MEMS accelerometer includes proof masses that move in-phase in response to a sensed linear acceleration. Self-test drive circuitry imparts an out-of-phase movement onto the proof masses. The motion of the proof masses in response to the linear acceleration and the self-test movement is sensed as a sense signal on common sense electrodes. Processing circuitry extracts from a linear acceleration signal corresponding to the in-phase movement due to linear acceleration and a self-test signal corresponding to the out-of-phase movement due to the self-test drive signal.

BACKGROUND

Numerous items such as smartphones, smart watches, tablets, automobiles,aerial drones, appliances, aircraft, exercise aids, and game controllersutilize sensors during their operation (e.g., motion sensors, pressuresensors, temperature sensors, etc.). In commercial applications,microelectromechanical (MEMS) sensors such as accelerometers andgyroscopes capture complex movements and determine orientation ordirection. For example, smartphones are equipped with accelerometers andgyroscopes to understand the movement of the smartphone, to augmentnavigation systems that rely on Global Position System (GPS)information, and to perform numerous other functions. Wearable devicesand internet-of-things (IoT) devices constantly measure movement andother characteristics of a person, animal, or electronic device. Inanother example, drones and aircraft determines orientation based ongyroscope measurements (e.g., roll, pitch, and yaw) and vehicles of alltypes implement assisted driving to improve safety (e.g., to recognizeskid or roll-over conditions).

MEMS sensors, such as MEMS accelerometers, exhibit certain sensitivitiesthat when left unaddressed effectively degrade the quality of sensingoperations, and even more so over time. Manufacturing tolerances,mechanical product wear, and operational electronic drift contribute toimprecise sense detection. During sensor manufacture as well asinstallation within end-use products, sensors may undergo product stressand deformation that cause sensitivity error effects. For example,capacitance-based MEMS sensors, such as accelerometers, may undergo asense capacitor gap change and sensing circuit gain changes over time.To combat these issues manufacturers attempt to compensate fordeviations from ideal or designed sensitivity. For example,manufacturing self-test procedures simulate real world operations byapplying inertial forces to sensors or electrostatic forces to sensorproof masses and measuring the responsive sensor behavior against anexpected result. The manufacturer then has the opportunity to adjust forsensitivity deviations.

In some instances, accelerometer products are tested for undesirablesensitivity changes by real-time monitoring. One of the ways to monitorMEMS accelerometer sensitivity changes in real time is by applying aself-test signal, designed to cause an expected physical movement ofproof mass components of the MEMS accelerometer relative to senseelectrodes, and measuring the proof mass acceleration response. Theself-test procedure attempts to mimic a motion in response to anacceleration.

SUMMARY

A microelectromechanical (MEMS) accelerometer includes a first proofmass, a second proof mass, first electrodes, and second electrodes. Thefirst electrodes are located adjacent to the first proof mass to sensemovement of the first proof mass along a first axis in response to alinear acceleration along the first axis. The second electrodes arelocated adjacent to the second proof mass to sense movement of thesecond proof mass along the first axis. The first proof mass and thesecond proof mass move in-phase along the first axis in response to thelinear acceleration along the first axis. A self-test drive circuitry iscoupled to the first proof mass and the second proof mass. The self-testdrive circuitry is configured to cause the first proof mass and thesecond proof mass to move out-of-phase along the first axis. Processingcircuitry is coupled to the first electrodes and the second electrodesand configured to receive a sense signal and to extract from the sensesignal a self-test signal corresponding to the out-of-phase movement ofthe proof masses and a linear acceleration signal corresponding to thein-phase movement of the proof masses.

A method of self-testing a microelectromechanical (MEMS) accelerometerincludes applying a self-test drive signal at a first polarity to afirst proof mass and applying the self-test drive signal at a secondpolarity to a second proof mass. The first polarity of the self-testdrive signal is opposite to the second polarity of the self-test drivesignal. Applying the first polarity of the self-test drive signal to thefirst proof mass and applying the second polarity of the self-test drivesignal to the second proof mass causes the first proof mass and thesecond proof mass to move out-of-phase along a first axis. The methodfurther includes detecting a sense signal comprising a linearacceleration signal and a self-test sense signal. The linearacceleration signal corresponds to the first proof mass and the secondproof mass moving in-phase and a self-test sense signal corresponds tothe first proof mass and the second proof mass moving out-of-phase. Theself-test sense signal and the linear acceleration signal are extractedfrom the sense signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present disclosure, its nature, andvarious advantages will be more apparent upon consideration of thefollowing detailed description, taken in conjunction with theaccompanying drawings in which:

FIG. 1 depicts an exemplary motion sensing system in accordance with atleast some embodiments of the present disclosure;

FIG. 2 depicts exemplary movements of proof masses of an out-of-planeMEMS accelerometer in accordance with at least some embodiments of thepresent disclosure;

FIGS. 3A-3D depict exemplary MEMS accelerometer proof massconfigurations for an in-plane MEMS accelerometer in accordance with atleast some embodiments of the present disclosure;

FIG. 4A depicts graphical representations of an exemplary MEMSaccelerometer self-test response with in-phase self-test, in accordancewith at least embodiments of the present disclosure;

FIG. 4B depicts graphical representations of an exemplary MEMSaccelerometer self-test response with out-of-phase self-test, inaccordance with at least some embodiments of the present disclosure;

FIG. 5 depicts exemplary self-test and sensing circuitry in accordancewith at least some embodiments of the present disclosure;

FIG. 6 depicts exemplary signals at different stages of self-test andsensing circuitry when no force is applied to MEMS accelerometer proofmasses, in accordance with at least some embodiments of the presentdisclosure;

FIG. 7 depicts exemplary signals at different stages of self-test andsensing circuitry in response to a linear acceleration applied to MEMSaccelerometer proof masses, in accordance with at least some embodimentsof the present disclosure;

FIG. 8 depicts exemplary signals at different stages of self-test andsensing circuitry in response to an out-of-phase self-test force appliedto MEMS accelerometer proof masses, in accordance with at least someembodiments of the present disclosure;

FIG. 9 depicts exemplary signals at different stages of self-test andsensing circuitry in response to both a linear acceleration and anout-of-phase self-test force applied to MEMS accelerometer proof masses,in accordance with at least embodiments of the present disclosure; and

FIG. 10 illustrates steps of an exemplary MEMS accelerometer self-testprocess in accordance with at least embodiments of the presentdisclosure.

DETAILED DESCRIPTION OF THE DRAWINGS

In some applications, such as in the automotive industry, consumersafety requirements encourage maintaining adherence to strict productspecifications throughout a product lifetime. Sensor product sensitivitychanges of MEMS sensor devices are monitored in real time and during thedevice in-field operations to identify changes in operation and providefor subsequent compensation or adjustment in light of those changes.This monitoring also allows ongoing reporting on the health of a productthroughout the product lifecycle. Notification of product performancedegradation, such as component displacements or wear, affords theopportunity to perform compensation for or adjustment of componentoperation. In accordance with some embodiments and methods, a real-time,power efficient, and robust approach to monitoring and compensating forsensitivity changes of an in-field MEMS sensor device is disclosed.

The MEMS sensor device may be a capacitance-based MEMS sensor, such aswithout limitation, a MEMS accelerometer. Performance of the MEMS sensordevice is maintained and improved over the lifetime of the device byimplementing a vibration-robust self-test processing circuitry. Further,the processing circuitry improves the functional safety of the MEMSsensor device over the device lifetime.

In some embodiments, self-test mechanisms are implemented to estimatethe MEMS sensor device sensitivity by measuring an amplitude response toan applied electrostatic force to facilitate compensation for anundesirable sensor response (e.g., gain change) or to raise awareness ofa potential electrical or mechanical component degradation, for example.The self-test routine stimulates an out-of-phase motion of proof masses(e.g., out-of-phase as compared to a typical in-phase movement of theproof masses in response to a measured force such as linearacceleration), generating a responsive signal distinguishable fromsignals generated by regular in-field device operations, such as linearacceleration or vibration-related signals. For example, in a MEMSaccelerometer the motion applied to the accelerometer proof masses bythe self-test routine is out-of-phase compared to the proof massresponse to a sensed linear acceleration. Respective MEMS accelerometerproof masses may be designed to move in a particular manner with respectto each other (e.g., an in-phase response to linear acceleration). Thesesame respective proof masses may be excited out-of-phase by theself-test motion, such that the out-of-phase self-test response can bedistinguished from the in-phase response to linear acceleration.

In response to a self-test signal applied to the proof masses of theMEMS sensor device, an electrostatic force sets the proof masses of thedevice in an out-of-phase motion. Even in the presence of an externallinear acceleration that also causes movement of the proof masses, thein-phase component due to linear acceleration can be distinguished andisolated from the out-of-phase self-test component. The sensed linearacceleration and the monitored self-test response are extracted forfurther processing, for example, to perform compensation and/or togenerate a monitoring signal (e.g., representative of sensor health).

In an exemplary embodiment of differential sensing of in-phase linearacceleration by a MEMS accelerometer, the sense signal induced by theout-of-phase self-test motion is a common-mode signal. Unlikeconventional self-test techniques where the self-test motion imparted onrespective proof masses results in similar respective directions ofmovement as with linear acceleration, the self-test motion is observableeven in the presence of an unknown linear acceleration or other in-phasevibration.

Exemplary processing circuitry of a differential sensing MEMSaccelerometer may detect a linear acceleration signal based on theopposite polarities (+/−) of changes in the values of the respectivecapacitors formed between MEMS sensor device proof masses andcorresponding sense electrodes due to linear acceleration, and maymonitor a self-test output signal based on a common polarity (+/+or −/−)of changes in the values of the respective capacitors formed between theproof masses and corresponding sense electrodes due to the self-testmovements. In some embodiments, the differential response and thecommon-mode response are effectively multiplexed at the sense capacitorsto effect power conservation and circuit component savings andsubsequently demultiplexed to extract each portion of the sensedresponse. The responses are further processed by separate circuit pathsafter demultiplexing/extraction. Alternatively, the differential (linearacceleration) response and the common-mode (self-test) response may besensed from the sense capacitors in parallel therefore avoidingmultiplexing and demultiplexing the responses, with a tradeoff of largercircuit footprint and increased power consumption because parallelsignal conversion (e.g., capacitance-to-voltage conversion) may requireadditional circuit components in the processing circuitry.

The common-mode response is processed to identify a sensitivity change(e.g., a change in mechanical or electrical response, such as a gainchange) in the system to facilitate safety monitoring and sensitivitycompensation. Operational parameters such as gain scaling, filtering,offset values, and sensitivity scaling can be modified based on thesensitivity change. In some embodiments, errors and warnings may beprovided to other circuitry, facilitating alternative compensation orcalculation techniques (e.g., based on outputs from other sensors orcomponents).

In some embodiments, drive carrier signals applied to respective proofmasses may have different signal characteristics (e.g., frequency and/orphase). For example, applying a frequency multiple (e.g., 2×) to asubset of the proof masses facilitates multiplexing of the linearacceleration and self-test signals (e.g., because during a portion of ameasurement cycle the proof mass drive signals are in-phase, and duringother portions the proof mass drive signals are out-of-phase). In someembodiments, the frequency of the periodic carrier signals is the same,but the signals are out of phase. In some embodiments, the drive carriersignals are periodic and the frequency of one periodic carrier signal isa multiple of the frequency of the other periodic carrier signal toachieve separated differential and common mode sensing.

FIG. 1 depicts an exemplary motion sensing system 100 in accordance withsome embodiments of the present disclosure. Although particularcomponents are depicted in FIG. 1, it will be understood that othersuitable combinations of sensors, processing components, memory, andother circuitry may be utilized as necessary for different applicationsand systems. In an embodiment as described herein, the motion sensingsystem may include at least a MEMS accelerometer 102 (e.g., a single- ormulti-axis accelerometer for measuring linear acceleration along one ormore axes) and supporting circuitry, such as processing circuitry 104and memory 106. In some embodiments, one or more additional sensors 108(e.g., MEMS gyroscopes, additional MEMS accelerometers, MEMSmicrophones, MEMS pressure sensors, and a compass) may be includedwithin the motion processing system 100 to provide an integrated motionprocessing unit (“MPU”) (e.g., including 3 axes of MEMS gyroscopesensing, 3 axes of MEMS accelerometer sensing, microphone, pressuresensor, and compass).

Processing circuitry 104 may include one or more components providingnecessary processing based on the requirements of the motion processingsystem 100. In some embodiments, processing circuitry 104 may includehardware control logic that may be integrated within a chip of a sensor(e.g., on a substrate or capacitor of a MEMS accelerometer 102 or othersensor 108, or on an adjacent portion of a chip to the MEMSaccelerometer 102 or other sensor 108) to control the operation of theMEMS accelerometer 102 or other sensors 108 and perform aspects ofprocessing for the MEMS accelerometer 102 or other sensors 108. In someembodiments, the MEMS accelerometer 102 and other sensors 108 mayinclude one or more registers that allow aspects of the operation ofhardware control logic to be modified (e.g., by modifying a value of aregister). In some embodiments, processing circuitry 104 may alsoinclude a processor such as a microprocessor that executes softwareinstructions, e.g., that are stored in memory 106. The microprocessormay control the operation of the MEMS accelerometer 102 by interactingwith the hardware control logic, and process signals received from MEMSaccelerometer 102. The microprocessor may interact with other sensors ina similar manner.

Although in some embodiments (not depicted in FIG. 1), the MEMSaccelerometer 102 or other sensors 108 may communicate directly withexternal circuitry (e.g., via a serial bus or direct connection tosensor outputs and control inputs), in an embodiment the processingcircuitry 104 may process data received from the MEMS accelerometer 102and other sensors 108 and communicate with external components via acommunication interface 110 (e.g., a SPI or I2C bus, in automotiveapplications a controller area network (CAN) or Local InterconnectNetwork (LIN) bus, or in other applications suitable wired or wirelesscommunications interfaces as is known in the art). The processingcircuitry 104 may convert signals received from the MEMS accelerometer102 and other sensors 108 into appropriate measurement units (e.g.,based on settings provided by other computing units communicating overthe communication bus 110) and perform more complex processing todetermine measurements such as orientation or Euler angles, and in someembodiments, to determine from sensor data whether a particular activity(e.g., walking, running, braking, skidding, rolling, etc.) is takingplace. In some embodiments, some or all of the conversions orcalculations may take place on the hardware control logic or otheron-chip processing of the MEMS accelerometer 102 or other MEMS sensors108.

In some embodiments, certain types of information may be determinedbased on data from multiple MEMS inertial sensors 102 and other sensors108, in a process that may be referred to as sensor fusion. By combininginformation from a variety of sensors it may be possible to accuratelydetermine information that is useful in a variety of applications, suchas image stabilization, navigation systems, automotive controls andsafety, dead reckoning, remote control and gaming devices, activitysensors, 3-dimensional cameras, industrial automation, and numerousother applications.

An exemplary MEMS accelerometer 102 may include one or more movableproof masses that are configured in a manner that permits the MEMSsensor to measure a desired force (e.g., linear acceleration) along anaxis. In some embodiments, the MEMS accelerometer may be configured forsimultaneous self-test and measurement of linear acceleration. The MEMSaccelerometer proof masses, sense electrodes, and sense circuitry of theMEMS accelerometer may perform differential sensing based on in-phasemovements of the proof masses in response to linear acceleration. Byapplying out-of-phase self-test movements to the proof masses, thesensing of the self-test movements is a common mode sensing. Themovements in response to linear acceleration and self-test aremechanically multiplexed based on the movement of the proof masses.Either through distinct sense circuitry (e.g., common-mode anddifferential coupled C2V circuitry) or demultiplexing of output signalsfrom common mode sense circuitry, the respective outputs due to linearacceleration and self-test can be isolated and processed to measure bothlinear acceleration and accelerometer sensitivity in real time.

FIG. 2 shows exemplary movement of proof masses of an exemplaryout-of-plane MEMS accelerometer in accordance with some embodiments ofthe present disclosure. The proof masses of FIG. 2 and subsequentfigures depict nonlimiting examples of MEMS accelerometer devices. FIG.2 shows exemplary proof masses of an out-of-plane accelerometer (e.g., az-axis accelerometer) in response to different applied forces inaccordance with some embodiments of the present disclosure. MEMSaccelerometer 200 is shown to include two proof mass components, a proofmass 210 and a proof mass 212, positioned about an axis of rotation 220within the plane of a MEMS layer of MEMS accelerometer 200. Each of theproof masses 210 and 212 is offset with respect to axis of rotation 220,such that a z-axis acceleration causes opposite ends of the respectiveproof masses to move in unison (i.e., in phase).

At 202, MEMS accelerometer 200 is shown with proof mass 210 and proofmass 212 in at rest in-plane configuration along axis of rotation 220.At 202, proof masses 210 and 212 are static, exhibiting no movementsince linear acceleration along the sense axis (e.g., z-axis) remainsabsent. At 204, MEMS accelerometer 200 is shown with proof mass 210 andproof mass 212 moving in-phase about axis of rotation 220 in response toa linear acceleration, caused by application of a linear accelerationforce (“F_(accel)”). In this configuration, movement of proof masses 210and 212 is in-phase along the linear acceleration direction.

At 206, MEMS accelerometer 200 is shown with proof mass 210 and proofmass 212 having an in-phase self-test signal applied to cause movementof the proof masses 210 and 212. As depicted at 206, the force(“F_(st)”) of the exemplary self-test movement (e.g., caused byelectrodes, not depicted) is in-phase about the axis of rotation 220similar to an applied linear acceleration. In the presence of an appliedlinear acceleration as depicted in 204, the movement of the proof massesdue to self-test and the movement of the proof masses due to linearacceleration may be difficult to distinguish.

At 208, MEMS accelerometer 200 is shown with proof mass 210 and proofmass 212 moved out-of-phase about axis of rotation 220 in response to aself-test signal. Because proof masses 210 and 212 are offset withrespect to the axis of rotation 220, they should normally rotate aboutthe axis of rotation 220 in-phase as depicted at 204 and 206 in responseto the in-phase external force imparted upon the proof masses. In theexemplary embodiment of 208 however, the self-test signals are appliedto cause the proof masses to move contrary to their designed response tolinear acceleration (e.g., respective self-test drive electrodes causemovements of the proof masses that are out-of-phase with respect totheir designed motion due to linear acceleration). In this manner, asdepicted at 230, the self-test and acceleration motions impartcountervailing movements on the respective proof masses, which can bemeasured and extracted for independent processing as described herein.

FIGS. 3A-3D show exemplary MEMS accelerometer proof mass configurationsfor an in-plane MEMS accelerometer in accordance with some embodimentsof the present disclosure. With reference to FIGS. 3A-3D, each figureshows MEMS accelerometer 300 including two proof masses (PM1 and PM2)and four sets of electrodes, with two sets of electrodes positionedadjacent to each respective proof mass. Each pair of electrodes includesan electrode with a positive polarity (a positive electrode) and anelectrode with a negative polarity (a negative electrode). For example,a pair of self-test drive electrodes includes a positive self-test driveelectrode and a negative self-test drive electrode, and a pair of senseelectrodes includes a positive sense electrode and a negative senseelectrode.

In continued reference to FIGS. 3A-3D, in some embodiments, two sets ofthe four sets of electrodes are self-test drive electrodes and aremaining two sets of the four sets of electrodes are sense electrodes.A first pair of sense electrodes is generally located adjacent to afirst proof mass and a second pair of sense electrodes is generallylocated adjacent to a second proof mass. A first pair of self-testelectrodes is generally located adjacent to a first proof mass and asecond pair of self-test electrodes is generally located adjacent to asecond proof mass.

In FIGS. 3A-3D, a positive sense electrode of the first pair of senseelectrodes is generally positioned on a first adjacent side of the firstproof mass in a first direction along an axis and a negative senseelectrode of the first pair of sense electrodes is generally positionedon an adjacent side of the first proof mass opposite to the firstadjacent side of the first proof mass, in a direction opposite to thefirst direction along the same axis. A positive sense electrode of thesecond pair of sense electrodes is generally positioned on a firstadjacent side of the second proof mass in a first direction along anaxis and a negative sense electrode of the second pair of senseelectrodes is generally positioned on an adjacent side of the secondproof mass opposite to the first adjacent side of the second proof mass,in a direction opposite to the first direction along the same axis.

In FIGS. 3A-3D, a positive self-test drive electrode of the first pairof self-test drive electrodes is generally positioned on a firstadjacent side of the first proof mass in a first direction along an axisand a negative self-test drive electrode of the first pair of self-testdrive electrodes is generally positioned on an adjacent side of thefirst proof mass opposite to the first adjacent side of the first proofmass, in a direction opposite to the first direction along the sameaxis. In some embodiments, the axis is in plane with a MEMS layer ofMEMS accelerometer 300. In FIG. 3D, a positive self-test drive electrodeof the second pair of self-test drive electrodes is generally positionedon an adjacent side of the second proof mass, at a side opposite to alike side of the first adjacent side of the first proof mass and anegative self-test drive electrode of the second pair of self-test driveelectrodes is generally positioned on an adjacent side of the secondproof mass, at a like side of the first adjacent side of the first proofmass. In FIG. 3D, the polarity of the second self-test drive electrodepair is opposite to the polarity of the first test drive electrode pair.Each pair of self-test drive electrode applies a self-test drive signalto a respective proof mass. In response to the application of theself-test drive signal to the first and second proof masses, the proofmasses move out-of-phase relative to one another.

More specifically, as shown in each configuration of FIGS. 3A-3D, MEMSaccelerometer 300 includes a proof mass (PM1) 302 and a proof mass (PM2)304 and four sets of electrodes including a pair of self-test driveelectrodes 310, a pair of sense electrodes 312, a pair of senseelectrodes 314, and a pair of self-test drive electrodes 316. In eachconfiguration, a first pair of sense electrodes and a first pair ofself-test drive electrodes are generally located adjacent to proof mass302 and a second pair of sense electrodes and a second pair of self-testdrive electrodes are generally located adjacent to proof mass 304. Eachpair of electrodes includes a positive electrode and a negativeelectrode. Self-test drive electrodes 310 includes a positive self-testdrive electrode 310 a and a negative self-test drive electrode 310 b;sense electrodes 312 includes a positive sense electrode 312 a and anegative sense electrode 312 b; sense electrodes 314 includes a positivesense electrode 314 a and a negative sense electrode 314 b; andself-test drive electrodes 316 includes positive self-test driveelectrode 316 a and negative self-test drive electrodes 316 b.

In FIGS. 3A-3C, all positive sense electrodes, 312 a and 314 a, and allpositive self-test drive electrodes, 310 a and 316 a, are locatedadjacent to and on the left side of respective proof masses 302 and 304.Also in FIGS. 3A-3C, all negative sense electrodes, 312 b and 314 b, andall negative self-test drive electrodes, 310 b and 314 b, are locatedadjacent to and on the right side of respective proof masses 302 and304. In FIG. 3D, self-test drive electrode 310 a and positive senseelectrode 312 a are located adjacent to and on the left side of proofmass 302 and negative self-test drive electrode 310 b and negative senseelectrode 312 b are located adjacent to and on the right side of proofmass 302. Positive sense electrode 314 a is located on the left side andadjacent to proof mass 304 and negative sense electrode 314 b is locatedon the right side and adjacent to proof mass 304. In contrast to FIGS.3A-3C, in FIG. 3D positive self-test drive signal 316 a is located onthe right side of and adjacent to proof mass 304 and negative self-testdrive signal 316 b is located on the left side of and adjacent to proofmass 304. The polarity of self-test drive electrodes 316 is opposite tothe polarity of self-test drive electrodes 310 in FIG. 3D.

Depending on the applied forces, each of the proof masses 302 and 304 ofMEMS accelerometer 300 can move separately and distinctly relative toone other and in-phase and out-of-phase relative to a common axis.Differential sensing and common-mode sensing are performed based on themovements of proof masses 302 and 304. In an exemplary embodiment,differential sensing is performed in response to in-phase movements ofthe proof masses in a direction consistent with linear acceleration.Common-mode sensing is performed in response to out-of-phase movementsof the proof masses.

In the configuration 310 of FIG. 3A, similar to a correspondingout-of-plane configuration at 202 of FIG. 2, proof masses 302 and 304are static in the absence of any linear acceleration force, self-testforce, or vibration. In the configuration 320 of FIG. 3B, similar to acorresponding out-of-plane configuration at 204 of FIG. 2, proof masses302 and 304 move in-phase or in unison, as shown by arrows 342 and 344,respectively, in response to a linear acceleration, caused byapplication of a linear acceleration force (F_(accel)). In thisconfiguration, differential sensing is performed in response to anin-phase proof mass movement in the presence of acceleration. Thein-phase movement of proof masses 302 and 304 is along the linearacceleration direction. As proof mass 302 moves towards sense electrodeand 312 b and away from sense electrode 312 a, the capacitance betweenproof mass 302 and sense electrode 312 b increases while the capacitancebetween proof mass 302 and sense electrode 312 a decreases. Similarly,as proof mass 302 moves towards sense electrode and 312 b and away fromsense electrode 312 a, the capacitance between proof mass 302 and senseelectrode 312 b increases while the capacitance between proof mass 302and sense electrode 312 a decreases.

In the configuration 330 of FIG. 3C, similar to a correspondingout-of-plane accelerometer configuration at 206 in FIG. 2, differentialsensing is achieved in response to the application of an in-phaseself-test drive signal to self-test drive electrodes 310 and 316. Inresponse to application of a common self-test drive signal to self-testdrive electrodes 310 and 316 (e.g., with the positive polarity self-testdrive electrodes 310 a and 316 a on the same side of the proof massesand the negative polarity and negative-test drive electrodes 310 a and316 a on the opposite side of the proof masses), the self-test driveelectrodes 310 and 316 cause movement of proof masses 302 and 304,respectively, in a manner similar to the manner in which accelerationcauses movement of proof masses 302 and 304. Proof masses 302 and 304move in-phase to an applied linear acceleration.

In the configuration 330, proof masses 302 and 304 move in unison towardrespective negative sense electrodes and away from respective positiveelectrodes. Proof mass 302 moves toward negative sense electrode 312 bwhile moving away from positive sense electrode 312 a and proof mass 304moves toward negative sense electrode 314 b while moving away frompositive sense electrode 314 a. In this respect, the capacitance formedbetween proof mass 302 and negative sense electrode 312 b increases, asdoes the capacitance formed between proof mass 304 and negative senseelectrode 314 b while the capacitance formed between proof mass 302 andpositive sense electrode 312 a decreases, as does the capacitance formedbetween proof mass 304 and positive sense electrode 314 a. Accordingly,sense electrodes 312 and 314 respond in a similar manner to the in-phaseself-test motion as they do to a response to linear acceleration.

In the configuration 340 of FIG. 3D, similar to a correspondingout-of-plane accelerometer configuration at 208 in FIG. 2, in responseto a self-test drive signal, an out-of-phase self-test force is appliedto proof masses 302 and 304 causing proof masses 302 and 304 to moveout-of-phase, as shown by the opposite direction of movement of theproof masses by arrows 350 and 352, respectively. In the configuration340 of FIG. 3D, the polarity of self-test drive electrodes 316 isflipped relative to the polarity of self-test drive electrodes 310 andthe polarity of self-test drive electrodes of the configuration 330,causing out-of-phase movement of proof masses 302 and 304 andcommon-mode sensing due to the unchanged polarity of sense electrodes314 relative to sense electrodes 312. Unlike the configuration 330, themotion of proof masses 302 and 304 are out-of-phase relative to thedirection of acceleration.

As previously discussed, in the configuration 330, the capacitanceformed between proof mass 302 and negative sense electrode 312 bincreases, as does the capacitance formed between proof mass 304 andnegative sense electrode 314 b. In 330, the capacitance formed betweenproof mass 302 and positive sense electrode 312 a decreases, as does thecapacitance formed between proof mass 304 and positive sense electrode314 a. Whereas, in the configuration 340, the opposite occurs in thatproof mass 302 moves toward sense electrode 312 b and away from positivesense electrode 312 a while proof mass 304 moves toward positive senseelectrode 314 a and away from negative sense electrode 314 b. Thecapacitance formed between proof mass 302 and negative sense electrode312 b increases while the capacitance formed between proof mass 304 andnegative sense electrode 314 b decreases. In the same manner, thecapacitance formed between proof mass 302 and positive sense electrode312 a decreases while the capacitance formed between proof mass 304 andpositive sense electrode 314 a increases. Accordingly, in theconfiguration 330, the sense electrodes perform differential sensing inresponse to an in-phase self-test motion, and in the configuration 340,the sense electrodes perform common-mode sensing in response to anout-of-phase self-test motion.

FIG. 4A shows graphical representations 400 of an exemplary MEMSaccelerometer self-test response with an in-phase self-test movement, inaccordance with at least some embodiments of the present disclosure.FIG. 4B shows graphical representations 450 of an exemplary MEMSaccelerometer self-test response with an out-of-phase self-testmovement, in accordance with at least some embodiments of the presentdisclosure.

FIG. 4A shows a graphical representation 402 of an in-phase self-testdrive signal applied to a MEMS accelerometer under test. Graphicalrepresentation 402 includes two self-test drive signals (ST1 and ST2),both shown, one overlaid by another, by 416. The self-test drive signalsare active at a frequency common to the frequency of the self-testmotion, as shown by the square wave shape portions of graph 416.Otherwise, the self-test drive signals are inactive as reflected at theflat portions of graph 416. The in-phase self-test drive signals, ST1and ST2, are indistinguishable at graph 416. Graphical representation402 further shows an acceleration (or vibration) signal at graph 418.The acceleration signal, at graph 418, has a sinusoidal shape in anexemplary embodiment. It is understood that for the purpose ofsimplicity, acceleration graph 418 is an approximated representation ofvibration behavior, which may be any suitable pattern of accelerationapplied to the MEMS accelerometer.

Graphical representation 404 shows a graph of the self-test operationoutput in response to the in-phase differential sensing of graph 402.The differential output 404 includes linear acceleration 420,superimposed by the self-test drive signal, shown as 426. Accordingly,the MEMS accelerometer self-test output and linear acceleration outputinterfere with each other at portions where both movements are active. Acommon mode output 406 in response to these linear acceleration signalsdoes not have any output 422.

FIG. 4B shows graphical representations of a MEMS acceleratorconfiguration similar to the configuration of FIG. 3D and configuration208 of FIG. 2. More specifically, FIG. 4B shows a graphicalrepresentation 408 of an anti-phase self-test drive signal applied to aMEMS accelerometer under test, in accordance with various embodimentsand methods of the disclosure. Graphical representation 408 includes two(anti-phase) self-test drive movements (ST1 and ST2), shown by signals432 and 434, respectively. The self-test output signals are active at acommon frequency during a self-test duration, shown by the square waveshape portions of graphs 434 and 432, and otherwise inactive duringnon-self-test durations, shown by the remaining flat portions of thesignals. Graphical representation 408 further includes an accelerationsignal at the self-test output signal, shown as signa1430. As shown bygraphical representations 410 and 412, the linear acceleration signaland the self-test signals are both observable and distinct within thesense signal.

Graphical representation 410 shows a differential portion 436 of a sensesignal in response to the combined movements depicted in 408. Thedifferential signal 436 corresponds to the in-phase movement 430 of theproof masses in response to linear acceleration. In the exemplaryembodiment of FIG. 4B, linear acceleration 430 and differential sensesignal 436 are generally sinusoidal-shaped for ease of illustration,although it will be understood that an applied acceleration may have anysuitable signal patterns and may not be periodic. As depicted in 410,the differential sense signal 436 corresponds only to the accelerationsignal is therefore observable without interference by the self-testsignal.

Graphical representation 412 shows a common mode portion 438 of a sensesignal in response to the combined proof mass movements depicted in 408.A self-test output is generated in response to the MEMS accelerometeranti-phase self-test input (anti-phase proof mass movement). Signal 438has a square wave shape portion corresponding to where the anti-phaseself-test drive signal is active and is otherwise inactive, as shown bythe flat portions of signal 438. Although particular self-test drivesignal patterns are depicted in FIGS. 4A and 4B, it will be understoodthat a variety of other patterns (e.g., non-square wave signal patterns,non-periodic, pseudo-noise, CDMA, etc.) may be applied as self-testdrive signals. In the exemplary embodiment of FIG. 4B, as depicted at412, because the self-test drive signal is out-of-phase with themovement due to linear acceleration, the common mode portion 438 of thesense signal includes only the out-of-phase self-test signal 438,without interference from the in-phase (differentially sensed) linearacceleration signal.

FIG. 5 depicts exemplary self-test and sensing circuitry in accordancewith at least some embodiments of the present disclosure. The self-testand sensing circuitry of FIG. 5 may be configured, in part or in whole,as processing circuitry. In FIG. 5, an exemplary MEMS accelerometerincluding the self-test and sensing circuitry includes a proof mass 502(PM1), a proof mass 504 (PM2), a pair of self-test drive electrodes 506,a pair of a sense electrodes 508, a pair of a sense electrodes 510, apair of self-test drive electrodes 512, self-test drive circuitry 514,carrier drive circuitry 544, and sensing circuitry 516, in accordancewith some embodiments of the disclosure. As previously noted, relativeto preceding figures, each pair of sense electrode and each pair ofself-test drive electrode includes a respective positive electrode and arespective negative electrode. Accordingly, self-test drive electrodes506 include negative self-test drive electrode 506 a and positiveself-test drive electrode 506 b; sense electrodes 508 include positivesense electrode 508 a and negative sense electrode 508 b; senseelectrodes 510 include positive sense electrode 510 a and negative senseelectrode 510 b; and self-test drive electrodes 512 include positiveself-test drive electrodes 512 a and negative self-test drive electrodes512 b.

PM1 and PM2 move in-phase in response to a sensed linear acceleration.As described herein (e.g., based on the respective polarity and locationof the applied self-test drive signals), self-test drive circuitry 514causes an out-of-phase movement onto PM1 and PM2. In the exemplaryembodiment of FIG. 5, the motion of PM1 and PM2 in response to thelinear acceleration and the self-test movement is sensed as a sensesignal on a shared set of sense electrodes, and thus, is effectivelymultiplexed on the sense signal. Processing circuitry extracts from thesense signal a linear acceleration signal corresponding to the in-phasemovement due to linear acceleration and a self-test sense signalcorresponding to the out-of-phase movement due to the self-test drivesignal.

Sense electrodes 508 a and 508 b are located adjacent to and on eitherside of PM1, in a manner similar to sense electrodes 312 a and 312 b,respectively, of configuration 340 of FIG. 3D. Sense electrodes 510 arelocated adjacent to and on either side of PM2, in a manner similar tosense electrodes 314 a and 314 b, respectively, of configuration 340 ofFIG. 3D.

Self-test drive circuitry 514 is shown to generate SelftestN (drive) andSelftestP (drive) signals. Carrier drive circuitry 544 is shown toinclude PM1_Drive and PM2 Drive signals. Sensing circuitry 516 is shownto include a C2V 522, a demodulator 524, a demultiplexer 526, aself-test monitor path 528, and an acceleration sense path 530.Self-test monitor path 528 is shown to include a self-test filter 538and a self-test digital signal processor (DSP) 540. Acceleration sensepath 538 is shown to include an acceleration filter 532, an accelerationDSP 534, and a gain/offset/sensitivity (GOS) block 536.

PM1, PM2, self-test drive electrodes 506, sense electrodes 508, senseelectrodes 510 and self-test drive electrodes 512 may collectively forma MEMS accelerometer 552, an example of a MEMS sensor product undergoingreal time and continuous self-test monitoring during the productoperational lifetime, in accordance with various disclosed embodimentsand methods. MEMS accelerometer 552 includes a MEMS layer including asuspended spring-mass system including proof masses that move withrespect self-test drive electrodes 506, sense electrodes 508, senseelectrodes 510 and self-test drive electrodes 512.

In some embodiments, in-phase movement of PM1 and PM2 includessimultaneous movement of the proof masses toward all the positive senseelectrodes or all the negative sense electrodes. An in-phase movementexample is a PM1 and a PM2 simultaneous movement toward an electrode ofeach of the electrodes 506, 508, 510 and 512 with a common polarity. Theout-of-phase movement of PM1 and PM2 includes movement of one of theproof masses, either PM1 or PM2, in a direction toward a correspondingpositive sense electrode (at an adjacent side) of the proof mass andsimultaneous movement of the other proof mass in a direction toward acorresponding negative sense electrode (an adjacent opposite side) ofthe proof mass.

As shown in the embodiment of FIG. 5, self-test drive circuitry 514 iscoupled to PM1 and PM2 through the SelftestN and SelftestP drivesignals, the negative and positive polarities of a “self-test drivesignal”, respectively. In FIG. 5, SelftestN drive signal is showncoupled to self-test drive electrode 506 a and to self-test driveelectrode 512 a and SelftestP drive signal is shown coupled to self-testdrive electrode 506 b and self-test drive electrode 512 b. The SelftestNand SelftestP drive signals are applied at opposite sides to PM1 andPM2, therefore, self-test drive circuitry 514 causes PM1 and PM2 to moveout-of-phase. More specifically, the negative polarity of self-testdrive signal, SelftestN signal, is applied to positive self-test driveelectrode 506 a adjacent to PM1 and to positive self-test driveelectrode 512 a adjacent to PM2 while the positive polarity of self-testdrive signal, SefltestP signal, is applied to negative self-test driveelectrode 512 b adjacent to PM2 and to negative self-test driveelectrode 506 b adjacent to PM1. The polarities of the self-test driveelectrodes 506 and 512 are opposite to cause an out-of-phase proof massmovement. But because the polarity of sense electrodes 508 and 510 arenot in opposite, the sensing of the out-of-phase movement is commonmode.

The sensed movements of the proof masses are represented as capacitancechanges between the proof masses and respective sense electrodes basedon an in-phase and an out-of-phase movement of PM1 and PM2. As earliernoted, this dual motion feature of PM1 and PM2 is effectivelymultiplexed in the form of combined sense signal include a linearacceleration signal and a self-test signal, implementing a designarchitecture with fewer circuit components and lower power consumption.In other embodiments (not depicted in FIG. 5), a second C2V amplifiersimilar to 522 may be connected for common mode sensing.

Sensing circuitry 516 is coupled to MEMS accelerometer 552 at the proofmass outputs through the SenseP and SenseN signals of self-test drivecircuitry 514. SenseP signal is coupled to sense electrodes 510 a and508 a and SenseN signal is coupled to sense electrodes 510 b and 508 b.In some embodiments, a capacitance to voltage (C2V) converter 522 ofprocessing circuitry 516 is a differential amplifier coupled, at apositive input, to the combined SenseP signal, and at a negative input,to the combined SenseN signal. In response to changes in the SenseN andSenseP signals, C2V 522 generates a differential output, represented bya sense signal, that is detected by sensing circuitry 516.

More specifically, C2V 522 generates the sense signal based on thedifferential inputs of the SenseN and SenseP signals, with embeddedlinear acceleration and self-test signals that are subsequentlydemultiplexed into two distinct signals, such as shown at graphs 410 and412, of FIG. 4B, respectively. As earlier described, the self-testoutput signal is generated in response to an out-of-phase movement ofPM1 and PM2 and the linear acceleration signal is generated in responseto the in-phase movement of PM1 and PM2.

In the embodiment of FIG. 5, C2V 522 is configured as acapacitance-to-voltage C2V) converter. The capacitance measured based onthe movements of PM1 and PM2 relative to respective sense electrodes, isconverted to a proportional voltage by the C2V converter and furtherprocessed by the processing circuitry of FIG. 5.

During the MEMS accelerometer in-field operation of the exemplaryprocessing circuitry of FIG. 5, a relative in-phase proof mass movementcauses differential sensing and a relative out-of-phase proof massmovement causes common-mode sensing. During PM1 and PM2 in-phasemovement, for example, PM1 is caused to move to the right toward arespective sense electrode located to the right of PM1, and PM2 is madeto simultaneously move to the right toward a respective adjacent senseelectrodes located to the right of PM2, such as shown by the directionof movement of PM1 and PM2 in FIG. 3B. This in-phase movement has theeffect of increasing the capacitance between each proof mass and arespective electrode positioned to the adjacent right of the proof massin response to a force (e.g., linear acceleration) applied to the proofmass. More specifically, the capacitance between PM1 and sense electrode508 b increases while the capacitance between PM1 and sense electrode508 a decreases, and the capacitance between PM2 and sense electrode 510b increases while the capacitance between PM2 and sense electrode 510 adecreases. The in-phase proof mass movement simultaneously decreases thecapacitance between each proof mass and a respective electrodepositioned to the adjacent left of the proof mass.

A relative out-of-phase movement of PM1 and PM2, for example, PM1 movingtoward sense electrode 508 b and away from sense electrode 508 a and PM2moving toward sense electrode 510 a and away from sense electrode 510 b,is shown by the direction of the arrows 350 and 352 associated with PM1and PM2 in FIG. 3D, respectively, and results in common-mode sensing inthe exemplary configuration of FIG. 5. That is, in response to the proofmass self-test drive signal (SelftestP signal and SelftestN signal)applied to self-test drive electrodes 506 a and 506 b, PM1 moves closerto sense electrode 508 b and the capacitance between PM1 and senseelectrode 508 b increases while the capacitance between PM1 and senseelectrode 508 a decreases. In the meanwhile, in response to the proofmass self-test drive signal applied to self-test drive electrodes 512 aand 512 b, PM2 moves closer to sense electrode 510 a and the capacitancebetween PM2 and sense electrode 510 a increases while the capacitancebetween PM2 and sense electrode 510 b decreases.

In an exemplary embodiment, the PM1_Drive and PM2_Drive signals areperiodic carrier signals. In some embodiments, the PM1_Drive andPM2_Drive signals have a common frequency. In some embodiments, thePM1_Drive and PM2_Drive signals are in phase and a frequency of thePM1_Drive and PM2_Drive signals is a multiple of a frequency of theother one of the PM1_Drive and PM2_Drive signals. In the examples tofollow, the PM2_Drive signal is presumed to have a frequency twice thatof the frequency of the PM1_Drive signal. As earlier noted, embodimentswith drive signals of different frequencies facilitate separatingdifferential sensing from common-mode sensing and can possibly beeffective power consumption measures.

PM1_Drive and PM2_Drive signals are similar to the SelftestN andSelftestP signals in that both sets of signals are drive signals, butunlike the SelftestN and SelftestP signals, each of the PM1_Drive andPM2_Drive signals acts as a respective carrier of the linearacceleration signal and the self-test signal, illustrative by theexample graphs of FIGS. 6-9. Additionally, unlike the SelftestN andSelftestP signals, the PM1_Drive and PM2 Drive signals do not causephysical movement of the proof masses PM1 and PM2.

Demodulator 524 receives the sense signal, output of C2V 522, anddemodulates the sense signal to remove the carriers, PM1_Drive and PM2Drive signals, outputting the raw sense signal due to proof massmovement (e.g., including embedded linear acceleration signal andself-test signals) for further processing.

Although not depicted in FIG. 5, in some embodiments the processingcircuitry of FIG. 5 may include additional circuitry such as ananalog-to-digital converter (ADC) for digitizing the analog signalsgenerated by C2V 522 or other as processed by other subsequentcircuitry. For example, an ADC may be implemented between demodulator524 and demultiplexer 526, or between demultiplexer 526 and each ofacceleration sense path 530 and self-test monitor path 528, or in eachof acceleration sense path 530 and self-test monitor path 528.

Demultiplexer 526 extracts the linear acceleration signal and theself-test (output) signal from the sense signal, for example, byselectively outputting the differential and common-mode components ofthe combined sense signal. Demultiplexer 526 may output the linearacceleration signal onto accelerometer sense path 530 (e.g., atparticular time periods associated with a demux clock 550) and outputthe self-test signal onto self-test monitor path 528 (e.g., at otherparticular time periods associated with the demux clock 550). In thegraphs of FIGS. 6-9, to follow, during the negative (or low) periods ofthe demux clock cycles, the linear acceleration signal is extracted ontoself-test monitor path 528 and during the positive (or high) periods ofthe demux clock cycles, the self-test signal is extracted ontoaccelerometer sense path 530. It is understood that alternatively, theself-test signal and the linear acceleration signals may be extracted atopposite half clock periods. It is also understood that references to aparticular polarity and polarity coupling herein are merely examplepolarities and opposite polarities and polarity couplings iscontemplated.

In some embodiments, self-test filter 538 of self-test monitor path 528is configured as a low pass filter to remove noise caused by frequencyharmonics that may be associated with sampling among other causes.Analogously, acceleration filter 532 of acceleration sense path 530 isconfigured as a low pass filter for a similar reason. It is understoodthat filters 532 and 538 may each be a suitably different type offilter. Each of DSPs 534 and 540 digitally processes respective filteredoutputs from filters 532 and 538. In some embodiments, DSP 540 generatesa sensor health monitor output 554, in common-mode sensing, in responseto detection of a degraded component or component feature, for example.In a nonlimiting example, output 554 may identify errors from themeasured sensitivity based on absolute changes (e.g., compared to abaseline) or changes over time (e.g., abrupt changes in sensitivity).

In some embodiments, the outputs of DSPs 540 and 534 are received by GOSblock 536. The gain, offset, and scaling parameters of GOS block 536collectively modify the measured value based on characteristics of theMEMS accelerometer. The output of GOS block 536 generates signal 546corresponding to a sensed linear acceleration, which may be furtherprocessed by other analog and/or digital processing circuitry.

In some embodiments, any of the self-test drive signal pattern and/orthe acceleration signal may correspond to code-division multiple access(CDMA) signals to further assist in distinguishing self-test andacceleration signals, for example, as is described in commonly ownedU.S. patent application Ser. No. 16/874,418, filed on May 14, 2020, andentitled “MEMS SENSOR MODULATION AND MULTIPLEXING”, which isincorporated by reference as though set forth in full herein.

In some embodiments, the self-test (output) signal may be recovered fromthe sense signal with other implementations. For example, notch filtersmay notch the signal at the self-test signal frequency. In someembodiments, the processing circuitry may include a second C2V where oneC2V, for example, amplifier 522, performs differential sensingexclusively and at all times and another C2V, simultaneously performscommon-mode sensing exclusively and at all times. In this configuration,a demultiplexer is not needed because the linear acceleration signal andthe self-test signal are separated at sensing, not embedded in a commonsense signal.

FIGS. 6-9 show exemplary signals at different stages of self-test andsensing circuitry, in response to different applied linear accelerationand self-test signals. Each of the figures of

FIGS. 6-9 includes six graphs, a graph of a PM1 signal (PM1_Drive inFIG. 5) and a PM2 signal (PM2_Drive in FIG. 5), a C2V output graph, ademodulated signal graph, a demux clock graph, a linear accelerationsignal graph, and a self-test signal graph. Further, in each figure, thePM2 signal is presumed to have a frequency twice that of the frequencyof the PM1 signal. In the first graph of each of the FIGS. 6-9 (top-mostgraph), “PM1” represents the signal behavior of PM1_Drive of FIG. 5 and“PM2” represent the signal behavior of PM2_Drive of FIG. 5. Each of thePM1 and PM2 signals is presumed a square wave shaped periodic signal. Itis understood that each of signals PM1 604 and PM2 602 may have asuitably different signal shape. PM1 and PM2 are further presumed tohave opposite polarity relative to one another and the frequency of PM2is presumed twice the frequency of PM1. In each figure, the first (top)graph shows PM1 with a solid line and PM2 is shown with a dashed line.

In each of FIGS. 6-9, the horizontal axis represents time inmilliseconds (ms) and the vertical axis represents a range of normalizedsignal amplitude values. In each figure, a time range is shown on thehorizontal axis from 0 ms to 0.05 ms in 0.005 ms intervals. Signalamplitude ranges are shown on the vertical axis and vary based on thesignal. For example, at graph 610, the signal amplitude ranges may varyacross figures and across graphs of a figure. Graph 610 of FIG. 6 andgraph 910 of FIG. 9 both show a graph of PM1 and PM2 yet in FIG. 6, thesignal amplitude ranges from −0.1 to +0.1 and in FIG. 9, the signalamplitude ranges from −0.2 to +0.2 and at graph 610, the signalamplitude range is −0.1 to +0.1 while at graph 620, the signal amplituderange is 0.2 to +0.2.

In a second top-most graph of each figure, “C2V output”, a graph of theoutput of C2V 522 (sense signal) is shown; the third top-most graph ofeach figure is a graph of the output of demodulator 526; the fourthtop-most graph of each figure is a graph of the demux clock 550; thefifth top-most graph of each figure is a graph of the linearacceleration signal (extracted from the sense signal in FIG. 5); and thelast graph of each figure is a graph of the self-test (output) signal(extracted from the sense signal in FIG. 5).

In the exemplary embodiment depicted in FIG. 6, the MEMS accelerator 552proof masses (proof mass 502 and proof mass 504 of FIG. 5, for example)are static and not in motion either due to linear acceleration or aself-test motion. Accordingly, the proof mass 502 output and the proofmass 504 output are not modulated by a self-test signal or anacceleration signal.

With continued reference to FIG. 6, at graph 610 (output of C2V 522),during the periods when PM1 604 is high and PM2 602 is low or viceversa, the output of C2V 522 is at a maximum high (shown at 612 of graph610), +0.2, and a maximum low (shown at 614 of graph 610), −0.2,respectively, a differential of 0.1 and −0.1, i.e., +0.1 minus(+0.1)=+0.2, and −0.1 minus (−0.1)=−0.2, respectively. Because PM2 602has double the frequency of PM 604, where PM1 604 and PM2 602 have thesame amplitude (both are at −0.1 or both are at +0.1), the net output iszero, as shown at 616 of graph 610. Accordingly, graph 610 is made oftwo essential portions, a differential portion where the frequency ofPM2 602 is double that of PM1 604 and graph 610 is 0, at 616,common-mode carrier, and another portion where the C2V output is adifferential output at 612 and 614, a differential carrier.

The output of C2V 522 is demodulated (by demodulator 524) with PM2 602to generate a demodulated signal with behavior shown by graph 620. WherePM2 602 is high (at 0.1) and graph 610 is at 0 (shown at 616), thedemodulated signal is at high (at 0), as shown by 622 at graph 620,where PM2 602 is low and graph 610 is at +0.2, graph 620 is low, shownat 624, at graph 620, and where PM2 602 is high or low and graph 610 isat 0, graph 620 is at high, as shown at 626 at graph 620. Graph 630shows the behavior of the demux clock 650. At high half periods of demuxclock 650, shown at 632 of graph 630, acceleration signal 642, at graph640, is output by demux 626 and at low half periods of demux clock 650,shown at 634 of graph 630, self-test signal 652, at graph 650, is outputby demux 626. Both the acceleration signal 642 and self-test signal 652are flat at 0, due to the inactivity of proof masses 502 and 504.

In FIG. 7, the proof mass 502 and proof mass 504 movements areconsistent with the movement of the MEMS accelerometer due to linearacceleration and sensing is performed on the differential path, path 530in FIG. 5. A (linear) accelerometer signal 706, shown at graph 700,modulates both PM1 704 and PM2 702 and the way the signals add togetherin response to an in-phase movement of proof masses 502 and 504.Accordingly, PM1 704 and PM2 702, proof mass 502 and proof mass 504outputs, respectively, move together, enveloped in the same envelope ofaccelerometer signal 706. When the frequencies of PM1 704 and PM2 702align, their sums are differential values, as shown at 714 of graph 710.When PM1 704 and PM2 702 overlap, their sum is 0, as shown at 716 ofFIG. 710. The amplitude of the differential portions of C2V 522 changein accordance with the amplitude of accelerometer signal 706, anenveloped amplitude change. The demodulated C2V output with PM2 702 isshown at graph 720. As shown by graph 720, the demodulation outputfollows the amplitude of acceleration signal 706. Demux clock 550, shownat graph 730, switches between a differential output, shown at graph 740and a common-mode output, shown at graph 750. The common-mode output—thesensed self-test signal—has a value of 0 because no self-test signal isapplied in the configuration of

FIG. 7 and the differential output is the sensed acceleration signal asshown at graph 740.

FIG. 8, a self-test only (no acceleration) configuration, proof masses502 and 504 are moved out-of-phase. Therefore, sensing is performed onthe common-mode path, path 528, and not the differential path, path 530.Whereas if the movement of the two proof masses were in the samedirection as the linear acceleration, such as shown and discussedrelative to FIG. 7, sensing is performed on the differential path.

In FIG. 8, PM1 804 and PM2 802 (shown at graph 800) have the samefrequency but their movements are lined up differently because theself-test drive signals force proof masses 502 and 504 to move inopposite directions. Accordingly, at the output of C2V 522, shown bygraph 810, the differential parts of graph 810 become 0, as shown at812, and the common-mode parts of graph 810, show the PM1 804 and PM2802 differences, as shown at 814. Accordingly, on path 530, no signal isobserved, as shown at graph 840, and on the self-test path, path 528,the self-test movement is observed, as shown at graph 850.

In the configuration of FIG. 9, the acceleration and the self-testsignals are both present. With reference to graph 900, where PM2 902 isperiodic but with a small magnitude, acceleration moves one of the proofmass (of proof masses 502 and 504) in a first direction but whileacceleration attempts to move the remaining proof mass in the samedirection, the self-test signal forces the remaining proof mass to movein a direction opposite to the direction of the acceleration movement(or the direction of the other proof mass movement) by a displacementamount smaller than the displacement amount the first proof mass movesthe direction of acceleration. Accordingly, while the periodic self-testmovement exist, but it is at a smaller amplitude amount because it is ina direction opposite to the acceleration movement direction. Therefore,when the self-test movement and acceleration movement are both present,both proof masses 502 and 504 move at a location very close to acorresponding sense electrode and the varying small magnitude signal toa large magnitude signal effect results in PM2 902, as shown at graph900. The large magnitude parts of PM2 902, as shown at 906, show thebehavior of PM2 902 when acceleration and self-test move together. Atthe same time, the magnitude of PM1 904 decreases significantly becauseself-test is working against acceleration, as earlier explained, shownat 908. The demodulated signal follows the amplitude envelope of PM2902, as shown at graph 920. At half periods 932 (of graph 930) of demuxclock 550, acceleration data is acquired. The behavior of the acquiredacceleration data is shown at graph 940. During other half periods 934of demux clock 550, self-test data is acquired with the behavior of theacquired self-test data shown at graph 950. Demultiplexer 526 canextract the two types of data because they are out-of-phase relative toone another.

FIG. 10 depicts exemplary steps for self-test operation of an exemplaryMEMS accelerometer in accordance with at least some embodiments of thepresent disclosure. Although FIG. 10 is described in the context of theparticular structures and components of the present disclosure, it willbe understood that the methods and steps described in FIG. 10 may beapplied to a variety of MEMS accelerometer designs, self-testtechniques, processing circuitry, and compensation techniques. Althougha particular order and flow of steps is depicted in FIG. 10, it will beunderstood that in some embodiments one or more of the steps may bemodified, moved, removed, or added, and that the flow depicted in FIG.10 may be modified.

The self-test operation of FIG. 10 is described with reference to theprocessing circuitry of FIG. 5 for the benefit of simplicity. It isunderstood that alternate processing circuitry suitable for carrying outthe steps of FIG. 10 may be employed. At step 1002 of the self-testoperation, a self-test drive signal, at a first polarity, is applied toa first proof mass. For example, in FIG. 5, SelftestN and SelftestPsignals are applied to PM1 at a certain polarity. Simultaneously, atstep 1004, the self-test drive signal is applied at a polarity oppositeto the first polarity to a second proof mass. For example, aSelftestN/SelftestP polarity is presumed applied to PM1, aSelftestP/SelftestN polarity is presumed applied to PM2. In response toapplying the self-test drive signal to PM1 and PM2, a sense signal isgenerated. At step 1006, the sense signal is detected. For example, thesense signal may be generated by C2V 522 and detected at the output ofC2V 522. The sense signal has two embedded signals, each is a functionof a distinct PM1-PM2 movement. That is, the detected sense signalcarries a linear acceleration signal in response to a PM1 and PM2in-phase movement and a self-test output signal in response to a PM1 andPM2 out-of-phase movement. Next, at step 1008, the linear accelerationsignal and the self-test (sense) signals are extracted from the sensesignal. For example, demultiplexer 526 outputs the linear accelerationsignal at first half periods of demux clock 550 cycles and outputs theself-test sense signal at second half periods of the demux clock 550cycles. The linear acceleration signal may be further processed by theacceleration sense path components and the self-test signal is furtherprocessed by the self-test monitor path components as earlier discussedand shown relative to preceding figures. The foregoing description andFIG. 10 describe a self-test operation in accordance with theimplementation of FIG. 5. In alternate implementations where two C2Vsare employed, one for acceleration sensing and another for self-testsensing, the sense signal is two distinct signals, each generated by adistinct C2V, alleviating the need for a demultiplexer and demuxclocking. The accelerometer and self-test signals can be detected inparallel.

The foregoing description includes exemplary embodiments in accordancewith the present disclosure. These examples are provided for purposes ofillustration only, and not for purposes of limitation. It will beunderstood that the present disclosure may be implemented in formsdifferent from those explicitly described and depicted herein and thatvarious modifications, optimizations, and variations may be implementedby a person of ordinary skill in the present art, consistent with thefollowing claims.

1.-22. (canceled)
 23. A microelectromechanical (MEMS) accelerometer,comprising: a first proof mass; a second proof mass; one or more firstelectrodes located adjacent to the first proof mass to sense movement ofthe first proof mass along a first axis in response to a linearacceleration along the first axis during a self-test time period; one ormore second electrodes located adjacent to the second proof mass tosense movement of the second proof mass along the first axis during theself-test time period, wherein the first proof mass and the second proofmass move in-phase along the first axis in response to the linearacceleration along the first axis; self-test drive circuitry coupled tothe first proof mass and the second proof mass, wherein the self-testdrive circuitry is configured to continuously cause the first proof massand the second proof mass to move out-of-phase along the first axisduring the self-test time period; and sensing circuitry coupled toreceive a sense signal from the one or more first electrodes and the oneor more second electrodes, wherein the sensing circuitry outputs aself-test signal corresponding to the out-of-phase movement of the proofmasses during the self-test time period, without a linear accelerationsignal portion in the presence of linear acceleration.
 24. The MEMSaccelerometer of claim 23, wherein a positive electrode of the one ormore first electrodes and a positive electrode of the one or more secondelectrodes are located in a first direction along the first axis, and anegative electrode of the one or more first electrodes and a negativeelectrode of the one or more second electrodes are located in anopposite direction from the first direction along the first axis. 25.The MEMS accelerometer of claim 24, wherein the out-of-phase movement ofthe first proof mass and the second proof mass comprises simultaneousmovement of the first proof mass towards a corresponding positiveelectrode and the second proof mass towards an associated negativeelectrode.
 26. The MEMS accelerometer of claim 23, further comprising:at least one first self-test drive electrode located adjacent to thefirst proof mass; and at least one second self-test drive electrodelocated adjacent to the second proof mass, wherein to continuously causethe first proof mass and the second proof mass to move out-of-phaseduring the self-test time period, the self-test drive circuitry providesa first self-test drive signal with a polarity to the at least one firstself-test drive electrode and provides a second self-test drive signalwith an opposite polarity to the at least one second self-test driveelectrode.
 27. The MEMS accelerometer of claim 23, wherein the firstproof mass and the second proof mass are located within a plane of aMEMS layer, and wherein the first axis is within the plane of the MEMSlayer.
 28. The MEMS accelerometer of claim 23, wherein the first proofmass and the second proof mass are located within a plane of a MEMSlayer, and wherein the first axis is perpendicular to the plane of theMEMS layer.
 29. The MEMS accelerometer of claim 23, further comprisingdrive circuitry coupled to the first proof mass and the second proofmass to provide a first periodic drive signal to the first proof massduring the self-test time period and a second periodic drive signal tothe second proof mass during the self-test time period.
 30. The MEMSaccelerometer of claim 29, wherein the first periodic drive signal andthe second periodic drive signal have the same frequency and have anopposite polarity.
 31. The MEMS accelerometer of claim 30, wherein thefirst periodic drive signal and the second periodic drive signal havethe same polarity and the same frequency in order to extract theself-test signal during the self-test time period.
 32. The MEMSaccelerometer of claim 23, wherein the sensing circuitry is furtherconfigured to identify an error in the MEMS accelerometer based on theself-test signal.
 33. The MEMS accelerometer of claim 23, wherein theself-test signal comprises code division multiple access (CDMA) signals.34. The MEMS accelerometer of claim 23, wherein the sensing circuitrycomprises common-mode sensing circuitry, further comprising differentialsensing circuitry coupled to receive the sense signal from the one ormore first electrodes and the one or more second electrodes, wherein thedifferential sensing circuitry outputs a linear acceleration signalcorresponding to the in-phase movement of the proof masses during theself-test time period, without a self-test signal portion in thepresence of the self-test drive circuitry continuously causing the firstproof mass and the second proof mass to move out-of-phase.
 35. Amicroelectromechanical (MEMS) accelerometer, comprising: a first proofmass; a second proof mass; one or more first electrodes located adjacentto the first proof mass to sense movement of the first proof mass alonga first axis in response to a linear acceleration along the first axisduring a self-test time period; one or more second electrodes locatedadjacent to the second proof mass to sense movement of the second proofmass along the first axis during the self-test time period, wherein thefirst proof mass and the second proof mass move in-phase along the firstaxis in response to the linear acceleration along the first axis;self-test drive circuitry coupled to the first proof mass and the secondproof mass, wherein the self-test drive circuitry is configured tocontinuously cause the first proof mass and the second proof mass tomove out-of-phase along the first axis during the self-test time period;and sensing circuitry coupled to receive a sense signal from the one ormore first electrodes and the one or more second electrodes, wherein thesensing circuitry outputs a linear acceleration signal corresponding tothe in-phase movement of the proof masses during the self-test timeperiod, without a self-test signal portion in the presence of theself-test drive circuitry continuously causing the first proof mass andthe second proof mass to move out-of-phase.
 36. The MEMS accelerometerof claim 35, wherein the sensing circuitry comprises differentialsensing circuitry, further comprising common-mode sensing circuitrycoupled to receive the sense signal from the one or more firstelectrodes and the one or more second electrodes, wherein thecommon-mode sensing circuitry outputs a self-test signal correspondingto the out-of-phase movement of the proof masses during the self-testtime period, without a linear acceleration signal portion in thepresence of linear acceleration.
 37. A microelectromechanical (MEMS)accelerometer, comprising: a first proof mass; a second proof mass; oneor more first electrodes located adjacent to the first proof mass tosense movement of the first proof mass along a first axis in response toa linear acceleration along the first axis during a self-test timeperiod; one or more second electrodes located adjacent to the secondproof mass to sense movement of the second proof mass along the firstaxis during the self-test time period, wherein the first proof mass andthe second proof mass move in-phase along the first axis in response tothe linear acceleration along the first axis; self-test drive circuitrycoupled to the first proof mass and the second proof mass, wherein theself-test drive circuitry is configured to continuously cause the firstproof mass and the second proof mass to move out-of-phase along thefirst axis during the self-test time period; and processing circuitrycoupled to the one or more first electrodes and the one or more secondelectrodes, wherein the processing circuitry is configured to receive asense signal during the self-test time period and to extract from thesense signal a self-test signal corresponding to the out-of-phasemovement of the proof masses during the self-test time period and alinear acceleration signal corresponding to the in-phase movement of theproof masses during the self-test time period, and wherein the self-testsignal does not include a response to an external vibration applied tothe MEMS accelerometer.
 38. The MEMS accelerometer of claim 37, whereinthe processing circuitry is further configured to modify a linearacceleration determined from the linear acceleration signal based on theself-test signal.
 39. The MEMS accelerometer of claim 38, wherein themodification is based on gain change associated with the movement of thefirst proof mass or the second proof mass.
 40. The MEMS accelerometer ofclaim 37, wherein the processing circuitry comprises a demultiplexer,wherein the demultiplexer receives the sense signal, and wherein thedemultiplexer extracts the self-test signal and the linear accelerationsignal from the sense signal.
 41. The MEMS accelerometer of claim 37,wherein the processing circuitry comprises sensing circuitry coupled toreceive the sense signal, and wherein the sensing circuitry outputs theself-test signal without a linear acceleration signal portion in thepresence of linear acceleration.
 42. The MEMS accelerometer of claim 41,wherein the sensing circuitry comprises common-mode sensing circuitryand the processing circuitry further comprises differential sensingcircuitry coupled to receive the sense signal, and wherein thedifferential sensing circuitry outputs the linear acceleration signalwithout a self-test signal portion in the presence of the self-testdrive circuitry continuously causing the first proof mass and the secondproof mass to move out-of-phase.